1. Field of the Invention
The present invention relates to an ignition timing control apparatus for a internal combustion engine for controlling knocking in the engine.
2. DISCUSSION OF BACKGROUND
FIG. 9 is a block diagram showing a conventional ignition timing control apparatus for an internal combustion engine.
In FIG. 9, a reference numeral 1 designates an acceleration sensor attached to the internal combustion engine to detect a vibration acceleration in the engine, a numeral 2 designates a frequency filter which passes only a frequency signal component having a high sensitivity to the knocking among the output signals of the acceleration sensor 1, and a numeral 3 designates an analog gate for blocking noises which disturb the detection of the knocking signal in the output signals from the frequency filter 2.
The analog gate 3 is opened and closed by instruction from a gate timing controller 4.
An output signal from the frequency filter 2 passed through the analog gate 3 is supplied to a noise level detector 5 and a comparator 6, which also receives the output of the noise level detector 5.
The noise level detector 5 is to detect the level of mechanical vibration noises of the engine except for the knocking signals
The comparator 6 compares the output voltage of the analog gate 3 with the output voltage of the noise level detector 5 to produce a knocking detection pulse.
An integrator 7 receives the knocking detection pulse of the comparator 6 to conduct integration processing of it, and produces an integrated voltage depending on the magnitude of the knocking signal. The integrated voltage is supplied to a phase shifter 8, which displaces the phase of a reference ignition signal in response to the output voltage of the integrator 7.
On the other hand, a rotation signal generator 9 generates an ignition signal in accordance with a predetermined ignition advance angle characteristic to supply it to a waveform shaping circuit 10.
The waveform shaping circuit 10 is adapted to shape the ignition signal of the rotation signal generator 9 and at the same time, controls the closing angle in current conduction in an ignition coil 12. A numeral 11 designates a switching circuit for interrupting or continuing the current conduction from the ignition coil 12 in response to the output signal of the phase shifter 8.
FIG. 10 shows frequency characteristic in the output signal from the acceleration sensor 1. In FIG. 10, a curve A designates the characteristic in a case where knocking does not take place, and a curve B designates the characteristic in a case where knocking takes place.
In the output signal of the acceleration sensor 1, there are included a knocking signal, and the other signals signal generated when the knocking takes place, such as mechanical noises of the engine other than the knocking signal, and various noise components carried on a signal transmission line such as ignition noises, and so on.
In comparing the curve A with the curve B in FIG. 10, it is understood that the knocking signal has a peculiar frequency characteristic. Although the distribution in frequency varies depending on engines to be used and the position of the acceleration sensor 1 to be attached to the engine, there is clear difference between the case that the knocking occurs and the case that knocking does not occur. Accordingly, by filtering a frequency component (the band component of the central frequency f.sub.o) of the knocking signal, the noises having the other frequency components are suppressed and the knocking signal can be effectively detected.
FIGS. 11 and 12 show operating waveforms for various elements shown in FIG. 9. FIG. 11 concerns mode where there takes place no knocking in the engine, and FIG. 2 concerns mode where there takes place knocking in the engine.
The operations of the ignition timing control apparatus as shown in FIG. 9 will be described.
When the internal combustion engine is actuated, the rotation signal generator 9 generates an ignition signal in response to an ignition timing characteristic which is previously determined. The ignition signal is then subjected to waveform-shaping to be transformed into an opening and closing pulse with a given closing angle by the waveform shaping circuit 10. The shaped ignition signal drives the switching circuit 11 through the phase shifter 8 to thereby turn on and off current-feeding to the ignition coil 12. When the current is interrupted, the engine is fired by an ignition voltage produced in the ignition coil 12. Vibrations in the engine caused in the operations of the engine are detected by the acceleration sensor 1.
When there is no knocking in the engine, the vibrations in the engine resulted by to the knocking do not occur. However, the mechanical noises and the ignition noises are carried on the signal transmission line at the time of ignition F, and they are contained in the output signal of the acceleration sensor 1 as shown in FIG. 11a. When the output signal is passed through the frequency filter 2, the mechanical noise components are fairly suppressed as shown in FIG. 11b. However, the output signal having a large ignition noise component is sometimes outputted even after being passed through the frequency filter 2 since the magnitude of the ignition noise component is large. In this case, the ignition noises are recognized as knocking signals. Therefore, the analog gate 3 is used to interrupt the ignition noises by closing its gate in a period from the ignition by the output of the gate timing controller 4 which is triggered by the output of the phase shifter 8 (FIG. 11c). As a result, only mechanical noises having a low level as indicated by A in FIG. 11d are remained in the output of the analog gate 3.
On the other hand, the noise level detector 5 responds to change of the peak value of the output signal of the analog gate 3. In this case, the noise level detector 5 has the characteristics capable of responding to a relatively slow change in the peak value of mechanical noises and generates a d.c. voltage slightly higher than the peak value of the mechanical noises (as indicated by B in FIG. 11d).
Accordingly, since the output of the noise level detector 5 is greater than the average peak value of the output signal from the analog gate 3 as shown in FIG. 11d, no output signal is produced from the comparator 6 for comparing both signals as shown in FIG. 11e, with the result that the noise signal is completely removed. Accordingly, since there is no output voltage from the integrator 7 as shown in FIG. 11f, phase angle (reference in phase between the input and output signals in FIG. 11g and h) given by the phase shifter 8 is also 0. Accordingly, the phase of opening and closing the switching circuit driven by the output signal of the phase shifter 8, i.e. the phase of the current intermittently produced in the ignition coil 12 is the same as the phase of the reference ignition signal as the output from the waveform shaping circuit 10, whereby the ignition timing corresponds to the reference ignition timing.
When the knocking takes place, the output of the acceleration sensor 1 contains the knocking signal with a certain time delay from the ignition timing as shown in FIG. 12a, and the signal after being passed through the frequency filter 2 and the analog gate 3 is such that the knocking signal is overlapped with tee mechanical noises as indicated by A in FIG. 2d. Of the signal passed through the analog gate 3, since the rising part of the knocking signal is steep, response for the output voltage of the noise level detector 5 is delayed with respect the knocking signal. As a result, the input signals to the comparator 6 respectively take the form as shown by A and B in FIG. 12d, whereby pulses are produced in the output of the comparator 6 as shown in FIG. 12e.
The integrator 7 integrates the pluses to thereby produce an integrated voltage as shown in FIG. 12f. Since the phase shifter 8 displaces the output signal (the reference ignition signal as by FIG. 12b) of the waveform shaping circuit 10 to the side of delay in tim in response to the output voltage of the integrator 7, the phase of the output signal of the phase shifter 8 is lagged with respect to the phase of the reference ignition signal of the waveform shaping circuit 10. With such lag in phase of the output of the phase shifter 8, the switching circuit 11 is actuated to have the phase as shown in FIG. 12h. Accordingly, there causes delay in ignition time to result a state that the knocking is suppressed. Thus, the optimum ignition timing controlling is carried out by keeping the state as shown in FIGS. 11 and 12.
Heretofore, there was clear difference in distribution of the output signal from the acceleration sensor 1 depending on the presence or absence of the knocking at a frequency band lower than 10 KHz. Accordingly, the central frequency (f.sub.o of the frequency filter 10 as shown in FIG. 10 had been determined to have 6 KHz-9 KHz.
However, in the conventional ignition timing control apparatus, there was a problem such that mechanical noises of the engine or noises due to abnormal combustion other than the knocking appeared in the range of 6 KHz-10 KHz owing to the characteristic of an engine to be used so that these noises overlap the detected knocking signal, by which it was difficult to obtain a knocking signal having a clear difference of voltage with respect to these noise signals through the output of the frequency filter 2.
FIG. 13 shows an example of data obtained by analyzing the detection signal of the acceleration sensor 1 by using a frequency analyizer.
FIG. 13A shows a case that there is no knocking in the engine, and FIG. 13B shows a case that there is knocking in the engine. In FIG. 13A, there is found distinct level of noises in FIG. 13A, and FIG. 13B shows the noises in a low level.
It is found in FIG. 13A that the distribution of the noises is within 5.5 KHz-8.5 KHz, and there is no noises in the region of 10 KHz or higher. On the other hand, in FIG. 13B, knocking signals are within the regions of 6 KHz-8 KHz, 11 KHz-13 KHz and 15 KHz-17 KHz where the above-mentioned noises are present. Namely, since the knocking signals and the noises are present in the same frequency bands in the region lower than 10 KHz, it is not suitable to detect the knocking signal. However, in the region higher than 10 KHz, there appears no noise and only the knocking signals appear, and accordingly, this region is suitable to be used for detecting the knocking signal since difference between the presence of the knocking signals and the absence of the same is distinguished.
The conventional acceleration sensor is fundamentally difficult in assuring its characteristics to detect accurately the knocking signals appearing in the region of 10 KHz or higher or it is difficult to be manufactured.
FIG. 14A is a front view of a resonance type acceleration sensor 20 with a screw portion formed by a meter screw (hereinbelow, referred to as an M screw or an metric screw) which is used to attached the sensor 20 to the engine. By fastening the meter screw portion 21 with the corresponding a female M screw formed in the engine, the fastening seat surface portion 22 of the sensor 20 is in close-contact with the surface of the engine.
FIG. 15 shows the characteristics of the acceleration sensor 20. FIG. 15A is a diagram showing a relation of exciting frequency to output signal, FIG. 15B is a diagram showing a relation of exciting force to resonance frequency f.sub.r, and FIG. 15c is a diagram showing a relation of exciting force to frequency band width f.sub.BW.
FIG. 15A shows a voltage produced from the acceleration sensor 20 when an exciting frequency is changed under the condition of the acceleration being constant. In FIG. 15A, the maximum frequency is referred to as the resonance frequency f.sub.r, and the width of the frequency region at the voltage level 3 dB lower than the voltage of the resonance frequency fr is referred to as a band with f.sub.BW.
In the resonance type acceleration sensor 20, the piezo-electric element is formed, for instance in a cantilever form, and accordingly, it is difficult to adjust the resonance frequency f.sub.r, especially, there was a problem that it was necessary to further precise adjustment of the resonance frequency and the sensor 20 should have a accurate construction in the region higher than 10 KHz. For instance, the size of the structural elements must be small, whereby machining and treatment of these elements become difficult. The number of steps of work is increased in order to meet the resonance frequency since the frequency of the knocking signal is variable depending on engines to be used.
Since the band width f.sub.BW is narrow, Q=f.sub.r /f.sub.BW is high. Accordingly, it responds excessively to the knocking signal, and the knocking signal can not accurately be detected. Particularly, there was a great influence when the engine is driven at a high speed, and reduction in controlling ability could not be neglected.
When observation was made by using an actural apparatus, it was found that Q should be about 10 or lower. However, in the conventional resonance type sensor, Q was 15 or higher.
FIG. 14B is a diagram of the cantilever type piezo-electric element, and FIG. 14C is a diagram showing the principle of the operation of the element. In FIG. 14B, a fixing base 23 supports an end of a vibration detecting element 24 of a cantilever type which is constituted by a piezo-electric element. The vibration detecting element 24 is formed by bonding two piezo-electric elements having polarization in the direction of thickness (a simbol l designates the direction of length) so that the pezo-electric elements are mutually in contact with the same polarity and one end of the bonded element is fixed to be in a cantilever structure (FIG. 14C).
As shown in FIG. 14C, when the fixing base 23 is caused to be vibrated downwardly, the vibrator is deflected upwardly, whereby the upper piezo-electric element 24 is compressed to produce a negative electric charge, and on the hand, the lower piezo-electric element 24 is pulled to produce a positive electric charge.
When the fixing base 23 is oscillated upwardly in FIG. 14C, the two piezo-electric elements 24 undergo a pulling force and a compressing force as contrary to the above-mentioned case, whereby the electric charges are produced in the different elements.
In accordance with the above-mentioned principle, the piezo-electric element 24 are caused resonance at the frequency f.sub.r, whereby they respond to the oscillation of the resonance frequency f.sub.r to thereby produce the electric charges.
FIG. 15B is a graphical representation in which the resonance frequency f.sub.r changes under the condition that the exciting frequency is constant and only acceleration speed is changed. The FIG. 15B shows that the resonance frequency f.sub.r is changed by changing the acceleration speed. It gives influence to detect the knocking signal.
FIG. 15C is a graphical representation in which the band width f.sub.BW is changed when the resonance frequency is constant and only the acceleration is changed. The FIG. 15c shows that the band width is changed by changing the acceleration speed. It gives influence to detect the knocking signal.
FIG. 16 shows a non-resonance type acceleration sensor 25 with a tapered screw portion which is used to attach it to the engine. The non-resonance type acceleration sensor 25 is attached to the engine by fastening the tapered screw portion 26 to a corresponding female screw portion formed in the engine so that the tapered screw portion 26 is entered in the female screw portion from the free end of it to the line indicated by a reference numeral 27. In other words, there is a small gap between the shoulder portion of the main body of the sensor 25 and the surface of the engine. The above-mentioned acceleration sensor 25 is disclosed in, for instance, Japanese Unexamined Utility Model Publication No. 13673/1983.
FIG. 17 shows the characteristic of the acceleration sensor 25, wherein the abscissa represents exciting frequency and the ordinate represents the magnitude of output signal. In FIG. 17, as the frequency increases, the magnitude of an output signal becomes large although the non-resonance type sensor shows no change of output signal with respect to the exciting frequency. In such acceleration sensor, there is a limitation at about 10 KHz. When the frequency is 10 KHz or higher, it can not be practically used. The reason is that the higher the frequency is, the stronger the resonance characteristic is, because the acceleration sensor 25 is fixed to the engine in a state that it is fastened at the fastening position 27 and there is a space gas between the shoulder of main body of the sensor 25 and the surface of the engine.